The presently disclosed subject matter relates to a pulse photometer for calculating the concentration of a light absorption material in blood of a subject (hereinafter, the concentration is often referred to as “blood light absorber concentration”), and a method for calculating the blood light absorber concentration by using the pulse photometer.
A pulse photometer is an apparatus which measures the blood light absorber concentration of the subject. Specifically, the living tissue of the subject is irradiated with light beams at a plurality of wavelengths which have different ratios of the blood light absorbances depending on the blood light absorber concentration. The intensities of the light beams at the wavelengths transmitted through or reflected from the living tissue are detected. The intensities at the wavelengths are changed in accordance with the pulsation of the blood in the subject. Therefore, temporal changes of the intensities at the wavelengths due to the pulsation are acquired in the form of a pulse wave signal. The amplitudes of pulse wave signals with respect to waveforms correspond to light attenuation variations with respect to the waveforms, respectively. The blood light absorber concentration calculated based on a ratio of light attenuation variations with respect to waveforms (for example, see Japanese Patent No. 4,196,209).
As an example of the blood light absorber concentration, known is the arterial oxygen saturation (hereinafter, referred to as the SaO2) which is used as an index of oxygenation of blood. In order to obtain the value of the SaO2, an invasive measurement must be performed. Therefore, the transcutaneous arterial oxygen saturation (hereinafter, referred to as the SpO2) which can be non-invasively calculated is widely used as the index. The SpO2 is measured by a pulse oximeter which is an example of a pulse photometer.
Ideally, the value of the calculated SpO2 is equal to that of the actual SaO2. However, it is known that the values are different from each other under certain conditions. In the case where the pulse wave signal has a low amplitude, particularly, the value of the SpO2 tends to be calculated to be higher than that of the actual SaO2. In this case, a situation where, even when the value of the SpO2 indicates the normal condition of the subject, the subject actually suffers hypoxemia is possibly caused.